This invention relates to chemical-mechanical polishing (CMP) system and, in particular, to a method and an apparatus in which slurry is recycled and rejuvenated.
CMP apparatus is used primarily for polishing or "planarizing" the front face or device side of a semiconductor wafer. A polishing step is performed one or more times during the process for making integrated circuits and provides several advantages. For example, step coverage is improved because the size of a step is decreased. Lithography, projecting an image onto a layer of photoresist, is improved by a flatter surface. If the layer of photoresist were uneven, there is a chance that part of the image would be out of focus. Thus, polishing a wafer improves patterning the photoresist and improves the quality of the resulting devices.
In a typical CMP apparatus, a semiconductor wafer is rotated against a rotating polishing pad while an abrasive and chemically reactive solution, slurry, is supplied to the rotating pad. A polishing pad is typically constructed in two layers overlying a metal platen with the less resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of a layer. The platens used for a polishing pad and for a polishing head are carefully machined to have optically flat, parallel surfaces.
A polishing pad is typically two or three times the diameter of the wafer being polished and the wafer is kept off-center on the pad to prevent grinding a non-planar surface into the wafer. The axis of rotation of the wafer and the axis of the rotation of the pad are parallel, but not collinear, to keep the front face of the wafer parallel with the back face. Other CMP apparatus use an oscillating pad or a continuous belt pad. The invention is described in conjunction with a rotating pad and is applicable to any type of pad.
The parameters of polishing, such as the downward pressure on the wafer, the rotational speed of the carrier, the speed of the pad, the flow rate of the slurry, and the pH of the slurry, are carefully controlled to provide a uniform removal rate, a uniform polish across the surface of a wafer, and consistency from wafer to wafer.
Slurries used for CMP can be divided into three categories, depending on their intended use: silicon polish slurries, oxide polish slurries, and metal polish slurries.
A silicon polish slurry is designed to polish and planarize bare silicon wafers. A silicon polish is typically composed of very small abrasive, such as silica (SiO2), alumina (Al2O3), or ceria (Ce2O3) particles, of typically 20-200 nanometers in diameter, suspended in a water-based liquid. A common silicon polishing slurry uses silica particles in a colloidal suspension. The proportion of particles in a slurry is typically from 1-15% by weight. The pH of the slurry is typically from 8.0-11.5 and is controlled by the addition of an alkali, such as NaOH, KOH, or NH4OH.
An oxide polish slurry is designed to polish and planarize a dielectric layer on a wafer, typically a layer of silicon dioxide. The dielectric layer is formed by techniques well known in the art, such as oxidation or chemical vapor deposition. An oxide polish slurry is typically composed of very small abrasive, such as silica, alumina, or ceria particles 50-1000 nanometers in diameter, suspended in a water based liquid. The proportion of the particles in an oxide polish slurry is typically 1-15% by weight. The pH is kept above 8 and is typically 10.0-11.5. The pH of a slurry is controlled by the addition of an alkali. U.S. Pat. No. 4,910,155 (Cote and Leach) describes procedures and materials for oxide polishing.
A metals polish slurry is designed to polish and planarize a conductive layer on a semiconductive wafer. The conductive layer is typically deposited on a dielectric layer and can be any one of several conductive materials such as tungsten, titanium, aluminum, copper, doped silicon, doped polysilicon, or a metal silicide layer. The dielectric layer typically has openings ("vias") that are filled with the conductive material is to provide a path through the dielectric layer to previously deposited layers. After the conductive layer is polished, only the conductive material in the vias remains in the dielectric layer.
A metals polish typically includes very small particles of abrasive, such as silica, alumina, or ceria having a diameter of 50-1000 nanometers and suspended in a water based liquid. The proportion of the particles in the slurry is typically 1-5% by weight and the pH is typically less than 5. The pH of a metals polish slurry is optionally controlled by the addition of organic acids such as potassium acetate, acetic acid, or citric acid. In addition to the organic acid, the slurry may include one or more oxidizing agents to remove the conductive material. Typical oxidizers include hydrogen peroxide, potassium ferricyanide, ferric nitrate, or mixtures thereof. U.S. Pat. No. 5,340,370 (Cadien) describes a metals polishing process and some slurries that have been developed for metals polishing.
Slurries for CMP are commercially available from such companies as CABOT, Cab-0-sil Division, Tuscola, Ill. and Rodel Inc., Newark, Del.
As a wafer is polished, the slurry and abraded materials tend to glaze the surface of the pad, making the pad slick and reducing the polishing rate. Polishing can produce stray particles from the pad material, the wafer itself, or elsewhere. When these by-products are of sufficient concentration to adversely affect the polishing, they should be removed.
The slurry also changes chemically during the polishing process, changing composition and pH. The pH typically changes in an unfavorable direction and, as a result, a wafer polishes more slowly at the end of the life of the slurry than at the beginning. If a slurry contains an oxidizer, the oxidizer is partially consumed in the polishing process.
Conditioning a polishing pad removes old slurry particles and abraded particles from the pad and refreshes the surface of the pad with new slurry. Conditioning a pad typically includes removing the glaze and producing a microscopic roughness on the surface of the pad. Scraping the pad with a sharp object or roughening the pad with an abrasive restores the pad surface.
In many CMP systems, especially those used by large volume semiconductor manufacturers, the slurry flows continuously onto the polishing pad. As the pad rotates, slurry is flung off the edge and carried away by a drain. Although a continuous flow of fresh slurry is beneficial and desirable, one must provide a large quantity of slurry.
U.S. Pat. No. 5,299,393 (Chandler) discloses a removable containment device or dam that surrounds a rotating polishing pad. The dam enables one to store slurry on the polishing pad and to use considerably less slurry to polish a wafer. Because the slurry is being used in batches, the polishing rate decreases continuously during polishing until the batch is replaced, then the polishing rate abruptly increases. Such changes make it difficult to characterize a process accurately.
A dam also limits how quickly a CMP system can polish wafers. At high rotational speeds of the pad, the slurry is driven to the outer edges of the pad, producing an uneven distribution of slurry on the pad and causing uneven polishing. To reduce the centrifugal effect, the pad must be rotated at slower speeds than without the dam. Slower rotation is undesirable because it reduces the polishing rate.
In addition to slurry, a large volume of rinse water is used to remove the slurry particles and chemicals from the wafer and the various pads and parts of the equipment. Rinse water is typically used to keep a pad wet between periods of polishing. Slurry is supplied only during actual polishing in order to minimize consumption. Rinse water is also used on a secondary polishing pad, known as a buff pad, to scrub particles of slurry and adherent chemicals from the wafer before the wafer is removed from the polisher.
U.S. Pat. No. 3,549,439 (Kaveggia et al.) discloses a chemical lapping apparatus in which a pump is used to remove lapping compound from a lapping plate surrounded by a ridge for retaining the lapping reagent atop the plate and pumps the lapping compound through a filter to a separate reservoir. The lapping compound dissociates when heated to chemically react with the workpiece. In the reservoir, chemicals are added to the solution to maintain the desired concentration of lapping compound. Another pump then pumps the adjusted solution back onto the plate. This patent is similar to the Chandler patent in that a ridge or containment device is used to dam the liquid, thereby keeping a set level of slurry or lapping compound on the polishing pad.
U.S. Pat. No. 4,459,781 (Li) discloses applying an abrasive slurry containing a mixture of particle sizes to a rotating polishing wheel and allowing centrifugal force to separate the particles by size. A workpiece is polished by moving from the outer edge of the wheel toward the center of the wheel, where the smallest particles are. This is effective only for slurries containing relatively large particles.
U.S. Pat. No. 5,478,435 (Murphy et al.) discloses a point-of-use slurry dispensing system for CMP apparatus in which concentrated slurry, a diluting agent and, in some instances, a third chemical, are delivered separately to a polishing pad and mixed on the pad or in a dispensing line just prior to use, for control over dilution, temperature, and chemical infusion. Mixing takes place immediately on the pad before the slurry is swept under the wafer or in a small section of plumbing immediately adjacent the rotating pad. The patent relates to on-pad mixing and does not discuss reducing the amount of slurry used, recycling the slurry, rejuvenating used slurry, or recycling rinse water.
The amount of slurry delivered to a polishing pad depends on the material being polished, among other variables, and can vary widely. Slurry can flow onto the polishing pad at 20-500 milliliters per minute, with a typical flow of about 200 ml/min. Many users try to minimize the flow since the slurry is fairly expensive. The flow of slurry onto the polishing pad and the resulting hydrodynamics of the slurry circulating under the wafer are important to high speed and uniform polish. Polishing typically takes two to three minutes per polish cycle and consumes 400 to 600 milliliters of slurry, based upon 200 ml/min flow rate. Consumption of slurry can be as high as 1500 ml per cycle based upon a flow of 500 ml/min.
In typical CMP systems, slurry and rinse water are not segregated, both being directed down a waste drain. The volume of rinse water used is typically more than thirty times the volume of slurry used and can be more than one hundred twenty times the volume of slurry consumed. In a semiconductor manufacturing plant producing 10,000 wafers per month with three separate CMP cycles, from 12,000 to 18,000 liters of slurry per month are consumed and sent to waste drain, mixed with over 180,000 liters, or more, of water. This large chemical consumption adds considerably to the adverse environmental impact of wafer fabrication and adds considerably to the cost of manufacture.
Although CMP slurry is expensive, the risk of damaging a wafer whose value is between $10,000 and $50,000 must be weighed against the cost savings achieved by using recycled slurry. As a practical matter, the risk of damage from recycled slurry cannot be greater than the risk of damage from fresh slurry. The semiconductor industry needs a new, highly reliable solution to reducing the cost of CMP slurry through an effective slurry reprocessing and reuse system. Additionally, the semiconductor industry needs a new, highly reliable solution to reducing the cost of rinse water in CMP.
In view of the foregoing., it is therefore an object of the invention to reduce the consumption of slurry in CMP apparatus.
Another object of the invention is to provide an on-line process for continuously recycling slurry in CMP apparatus.
A further object of the invention is to provide recycled slurry for CMP apparatus in which the risk of damage from the recycled slurry is no greater than the risk of damage from fresh slurry.
Another object of the invention is to rejuvenate slurry in polishing apparatus.
A further object of the invention is to provide an improved CMP process by recirculating slurry and by adding chemicals to rejuvenate the slurry.
Another object of the invention is to improve the uniformity and consistency of a CMP apparatus.
A further object of the invention is to reduce the cost of operating CMP apparatus.
A further object of the invention is to recycle slurry in CMP apparatus without a substantial change in process, materials, or equipment, other than consuming less materials.
Another object of the invention is to retain the advantages of a continuous flow of slurry across the polishing pad while recycling the slurry.
A further object of the invention is to recycle slurry without causing abrupt changes in the physical or chemical characteristics of the slurry.
Another object of the invention is to reduce the consumption of rinse water in CMP apparatus.